Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The current demand for high power laser systems is growing in the market place and many techniques and methods of increasing laser power have been designed and developed for government and industrial applications. Increasing the brightness of a laser beam allows scaling laser power to a few hundred kilo-watts. There are two distinctive methods of scaling a laser beam such as coherent beam combining (CBC) and incoherent beam combining (IBC). CBC is relatively difficult compared to the IBC beam combining technique due to the phase lock requirements in the CBC technique.
Some of the advantages of the CBC technique include better beam quality, a narrow spectral bandwidth and high brightness compared to the IBC scheme. One of the major IBC methods is increasing brightness of the laser beam by combining multiple beams of different wavelengths, called wavelength beam combination (WBC), in a one-dimensional (1D) or two-dimensional (2D) configuration. There exist approaches that demonstrate a WBC beam combining technique in a 2D configuration using laser sources combined with a first-order grating stack. This WBC concept trades high spatial brightness for relatively large spectral bandwidth by combing multiple bandwidths of the laser sources. In order to improve the brightness in both dimensions a first-order grating stack is used to overlap horizontal and vertical dimensions of the optical beam to improve the brightness in both dimensions. During the WBC technique, a first and second grating are used to combine and improve horizontal and vertical brightness of the laser sources respectively. The combined and improved laser output beam is an incoherent laser beam with a relatively broad spectral bandwidth compared to each laser source.
Also, a similar WBC approach is used exclusively in a fiber amplifier with passive phase control. In order to achieve a single coherent beam combining with a diffractive grating, it requires a feed-back system to passively lock the phase of each fiber amplifier. Still, using this method, this approach broadens the spectral bandwidth of the combined beam. As the number of combined laser sources increases the spectral bandwidth will grow too, making it more difficult to phase-lock all laser sources.
The difficulty in designing a passive coherent beam combining technique lays with the need to lock all phase and spectral overlap of laser sources that are being combined. Most all CBC techniques require a feedback system to passively or actively lock all phase of the laser sources. This would require a very complicated optical or electrical feedback system.
Another difficulty in building a 2D coherent beam combining system is bundling together all laser sources in scaling up to a very high power laser system. Typically, it is a geometrical constraint to mount all laser sources in a compact form to coherently combine all laser sources. Currently there exists a 2D wavelength beam combining scheme using a grating stack that has two diffractive gratings combining a horizontal and vertical direction separately.
FIG. 9 is a perspective view of a prior art single-wavelength laser beam impinging on a diffraction grating. Referring to FIG. 9, a single-wavelength (monochrome) laser beam 1 impinges on a diffraction grating 5 and creates a diffraction pattern of 0th order 4, 1st order 7 and 2nd order 8. Diffraction grating 5 has a grating pattern of multiple a circular-shaped pattern (e.g., with multiple concentric rings) or a spiral-shape or pattern. Diffraction grating 5 projects perfect rings of diffractive patterns 4, 7 and 8 by the circular or spiral pattern of the grating. The rings of the circular patterned grating or the spiral of the spiral patterned grating are an intrinsic property of diffraction grating 5. In order to create a perfect ring of the diffracted pattern the laser beam 1 needs to impinge on the center of the circular or spiral pattern of the diffraction grating 5. The multiple rings caused by diffractive grating 5 are coherent light diffracted from the laser beam 1. This means that the light beam 2 impinging on the center of diffractive grating 5 splits into three diffractive orders of the laser beam 1 where these resultant beams 6 are in the same frequency and constant phase with respect to each other. The coherent beam characteristic of the diffracted beam caused by the circular or spiral pattern of diffraction grating 5 only works with a laser beam 1 of a single wavelength.
FIG. 10 is a projected side view of FIG. 9. FIG. 10 shows multiple diffracted orders of diffraction grating 5. The side view shows multiple cone-shaped rings are formed. This illustration shows that a single-wavelength (monochrome) laser beam 1 can be diffracted to create three different orders of diffraction patterns as an example. The circular patterned diffraction grating 5 generates diffracted rings of a single-wavelength laser beam 1 as shown in FIG. 10 where it has a bright spot at the center of the patterned rings. However, the spiral patterned diffraction grating 5 generates a dark spot at the center of rings similar to a ‘donut’ hole shape. The ring patterns 4, 7 and 8 of the diffracted laser beams 6 can be designed or changed by using different patterns in the diffraction grating 5 or changing the grating parameters.
FIG. 11 is a perspective view of a prior art multiple-wavelength laser beam impinging on a diffraction grating. Referring to FIG. 11, a multiple-wavelength laser beam 11 impinges on the diffraction grating 5 where each wavelength of laser beam 11 will generate multiple rings of diffraction orders. For example, if the multiple-wavelength laser beam 11 contains two distinctive wavelengths of λ1 and λ2 of laser beam 12 is impinging on the diffraction grating 5 and each wavelength of λ1 and λ2 will generate multiple rings of diffraction orders of 0th, 1st and 2nd orders. The λ1 wavelength of the laser beam 11 will create the 0th order of spot 20, the 1st order of ring 19 and the 2nd order of ring 17. The λ2 wavelength of the laser beam 11 will create the 0th order of spot 20, the 1st order of ring 14 and the 2nd order of ring 18 where the wavelength of λ1 is shorter than λ2. In this case the diffractive rings of the wavelength λ1 is coherent to each other and it is incoherent to the other wavelength λ2. The multiple-wavelength laser beam 11 can create multiple orders of rings that are coherent to the same wavelength and incoherent to different wavelengths. The wavelength separation technique of using circular or spiral patterned diffraction gratings can be applied to optical signal transmission such as a wavelength-division-multiplex (WDM). Alternatively, it can be used to combine multiple wavelengths into a single optical beam using the reverse process.
FIG. 12 is a projected side view of FIG. 11. FIG. 12 shows multiple diffracted orders of the multiple-wavelength laser beam 11 diffracted by the circular or spiral pattern of the diffraction grating 5. The side view shows that multiple cone-shaped rings are formed by multiple-wavelength laser beam 11. This illustration shows that a multiple-wavelength laser beam 11 can be diffracted to create three different orders of diffraction patterns for each wavelength of two, namely λ1 and λ2. The circular patterned diffraction grating 5 generates diffracted rings of multiple-wavelength laser beam 11 as shown in FIG. 11 where it has a bright spot of the mixed laser beam 11 of wavelength of λ1 and λ2 at the center, and the multiple orders of patterned rings have a distinctive wavelength of λ1 or λ2. However, the spiral patterned diffraction grating generates a dark spot at the center of the diffracted rings like a ‘donut’ hole shape. The ring patterns of the diffracted laser beams 14, 17, 18, 19 and 20 can be designed or changed by different patterns of the diffraction grating 11 or its grating parameters.